The problem of preventing the accumulation of dust and dirt particles reaching the interior of a printing head through the openings in the head for a plurality of print needles has been dealt with by various means. In one method, such as set forth in U.S. Pat. No. 3,833,105, includes the covering of the opening with a two-part plate. Each of the parts has a number of semi-circular grooves which together form circular holes when the circular grooves are matched. It should be apparent that the matching of the parts of the plates requires high precision. Furthermore, in published German patent application No. 2,119,134 a plate is disclosed that functions as a dust cover and which is provided with a number of holes and is placed over the openings for the printing needles. Although this method does not require the fitting of different parts of a plate together, it is apparent that there is considerable requirement for the precision assembly of the cover.
The present invention relates to a method of providing a mosaic type printing head with printing needles and having a dust-proof cover at the opening in the device for the printing needles.
It is an object of the present invention to eliminate the precision requirements of known dust-proof covers or dust protective arrangements for printing needles.
It is another object of the present invention to provide a dust-proof cover for printing needles of a printing head in which the opening for the printing needles is covered with a relatively thin layer of a suitable material and thereafter the printing needles are activated repeatedly to punch holes in the thin layer. It is another object of the present invention to mold the thin layer and its adjoining parts surrounding the opening for the printing needles together, said layer and adjoining wall area being constituted of the same material.
It is another object of the present invention to provide a layer of material in the opening which is different from the material of the adjoining parts and wherein the layer of material is held in the grooves in the edges of said opening.